A positioning of a device is supported by various Global Navigation Satellite Systems (GNSS). These include for example the American Global Positioning System (GPS), the Russian Global Navigation Satellite System (GLONASS), the future European system Galileo, the Space Based Augmentation Systems (SBAS), the Japanese GPS augmentation Quasi-Zenith Satellite System (QZSS), the Locals Area Augmentation Systems (LAAS), and hybrid systems.
The constellation in GPS, for example, consists of more than 20 satellites that orbit the earth. Currently, each of the satellites transmits two carrier signals L1 and L2. One of these carrier signals L1 is employed for carrying a navigation message and code signals of a standard positioning service (SPS). The L1 carrier phase is modulated by each satellite with a different C/A (Coarse Acquisition) code. Thus, different channels are obtained for the transmission by the different satellites. The C/A code is a pseudo random noise (PRN) code, which is spreading the spectrum over a 1 MHz bandwidth. It is repeated every 1023 bits, the epoch of the code being 1 ms. The carrier frequency of the L1 signal is further modulated with navigation information at a bit rate of 50 bit/s. The navigation information comprises inter alia ephemeris and almanac parameters. Ephemeris parameters describe short sections of the orbit of the respective satellite. Based on these ephemeris parameters, an algorithm can estimate the position of the satellite for any time while the satellite is in the respective described section. The almanac parameters are similar, but coarser orbit parameters, which are valid for a longer time than the ephemeris parameters. The navigation information further comprises for example clock models that relate the satellite time to the system time of GPS and the system time to the Coordinated Universal Time (UTC).
A GPS receiver of which the position is to be determined receives the signals transmitted by the currently available satellites, and it detects and tracks the signals used by different satellites based on the different comprised C/A codes. Then, the receiver determines the time of transmission of the code transmitted by each satellite, usually based on data in the decoded navigation messages and on counts of epochs and chips of the C/A codes. The time of transmission and the measured time of arrival of a signal at the receiver allow determining the pseudorange between the satellite and the receiver. The term pseudorange denotes the geometric distance between the satellite and the receiver, which distance is biased by unknown satellite and receiver offsets from the GPS system time.
In one possible solution scheme, the offset between the satellite and system clocks is assumed known and the problem reduces to solving a non-linear set of equations of four unknowns (3 receiver position coordinates and the offset between the receiver and GPS system clocks). Therefore, at least 4 measurements are required in order to be able to solve the set of equations. The outcome of the process is the receiver position.
Similarly, it is the general idea of GNSS positioning to receive satellite signals at a receiver which is to be positioned, to measure the pseudorange between the receiver and the respective satellite and further the current position of the receiver, making use in addition of estimated positions of the satellites. Usually, a PRN signal which has been used for modulating a carrier signal is evaluated for positioning, as described above for GPS.
A GNSS positioning can be performed in different positioning modes.
A first mode is a standalone mobile station based GNSS positioning. In this mode, the GNSS receiver receives signals from GNSS satellites. The GNSS receiver or an associated mobile device—referred to in common as mobile station—decodes navigation data directly from the satellite signals and calculates from these signals and the navigation data the position of the mobile station and other location information without any additional information from other sources.
A second mode is a network-assisted mobile station based GNSS positioning. In this mode, the GNSS receiver is associated to a mobile communication device. The GNSS receiver can be integrated into the mobile communication device or be an accessory for the mobile communication device. GNSS receiver and mobile communication device are referred to in common as mobile station. A mobile communication network provides assistance data, which is received by the mobile communication device. The assistance data can comprise for example ephemeris, position and time information. The assistance data can be used by GNSS receiver to improve its performance when acquiring and tracking satellite signals. Alternatively or in addition, the assistance data can be used at the mobile station in calculating the position of the mobile station and other location information. With provided assistance data, for example, it may not be required to decode the navigation information in tracked satellite signals.
The third mode is a network-based mobile station assisted GNSS positioning. For this mode, the GNSS receiver is associated as well to a mobile communication device. GNSS receiver and mobile communication device are referred to in common as mobile station. In this mode, a mobile communication network provides at least acquisition assistance and time information via the mobile communication device to the GNSS receiver for supporting the satellite signal measurements. The mobile station only performs signal measurements, though, and reports the measurements back to the network for position calculation.
The second and the third mode are also referred to in common as assisted-GNSS (A-GNSS). Assisted GNSS thus means that if the technical prerequisites are met, a mobile communication network is able to provide a GNSS receiver with assistance data, like time and navigation model, which allows the receiver to obtain a position fix in a shorter time and in more challenging signal conditions.
A network server, which generates assistance data and/or calculates position solutions for A-GNSS, can be for example the Serving Mobile Location Center (SMLC) server.
Regardless of the positioning mode, the SMLC can assist the mobile station for instance with accurate time assistance in order to speed up the Time-To-First-Fix and to increase the sensitivity of the mobile station to acquire very weak satellite signals. Accurate time assistance may be delivered from the SMLC to the mobile station as a time relation between the cellular system frame time, which is referred to as System Frame Number (SFN), and a satellite system time. In order words, the SMLC gives the mobile station information what GPS system time was/is/will be on a certain specific frame number identified by the SFN. Typically, the SFN that is taken as the timing reference is selected close to the current SFN in order to minimize any errors caused by base station clock drift in extrapolating the time estimate to real time. An estimate of the base station clock drift with respect to the GPS time can optionally be included in the assistance data to mitigate the clock drift based errors.
Typically, the mobile station also solves accurate GNSS time along with the position and possibly also calculates a relation between the GNSS system time and cellular SFN. The time relation can be used locally in the mobile station to maintain or recover accurate time with the help of SFN. Alternatively or in addition, the relation can be sent to an SMLC, for SMLC based maintenance of time relations and even for distribution to other mobile stations.
In both mobile station based modes, a network server can also request more comprehensive location information determined by the mobile station. Such location information may be used for example for location based services requested by the mobile station or by another entity, such as a friend finding service or a Yellow Pages service. In this case, the mobile station will send the location information that it has determined in the position calculations to the network server using a dedicated location information message or location information elements (IE) in another message. Location information elements are defined in different cellular standards and typically comprise:    1. Position information in the World Geodetic System 1984 (WGS-84) coordinate frame, including latitude, longitude and altitude    2. Position uncertainty ellipse    3. Velocity information, including velocity components in a local coordinate frame: heading, heading uncertainty, horizontal speed, horizontal speed uncertainty, vertical speed, vertical speed uncertainty    4. Cellular frame time—satellite time associations    5. Reference time, that is, the time when the location information was calculated and which is to be used as the reference for the cellular frame time—satellite time associations. The reference time is preferably given either in units of seconds [s] or milliseconds [ms].